1. Field of the Invention
The present invention relates to a sandwich structure to be used as a structural member of aircraft, rolling stock, automobiles and ships, and a method of repairing such a sandwich structure.
2. Description of the Related Art
Lightweight, reinforced metal plates of aluminum alloys have been used for forming structural members of aircraft and the like. Recently, reinforced, plate structures of a composite material and sandwich structures formed by laminating a honeycomb core between surface plates of a composite material have been prevalently used. The surface plates of a composite material are bonded to the honeycomb core with adhesive films. Usually, cell elements of a honeycomb core are bonded together with a foaming adhesive that foams in a hot pressure molding process.
When a surface plate of a composite material is damaged and needs repairing, it is general to repair the damaged surface plate by scraping off a damaged part of the surface plat, laminating adhesive layers and prepreg sheets formed by impregnating a fiber structure with a resin in a recess formed by scraping off the damaged part so as to fill up the recess, and applying heat and pressure to the layers of the prepreg sheets and the adhesive to set the layers of the prepreg sheets.
When a sandwich structure having a honeycomb core sandwiched between surface plates is damaged, it sometimes occurs that water leaks into the sandwich structure through the periphery of the sandwich structure or cracks formed in the surface plate and water collects in the sandwich structure. The water collected in the honeycomb structure diffuses throughout the sandwich structure through the foam adhesive layers bonding together the cell elements of the honeycomb core. The adhesive film used for bonding together the surface plates and the honeycomb core includes a glass fabric, a cotton fabric or a meshed carrier of synthetic fibers. Therefore, small gaps are formed between the surface plates and the honeycomb core and it is possible that the water spreads through the gaps. If water collects in the honeycomb core and the honeycomb core is made of a metal, such as an aluminum alloy, the honeycomb core will be corroded and the surface plates will separate from the honeycomb core.
Even if the honeycomb core is made of a nonmetallic material, water collected in the honeycomb core repeats freezing and evaporation according to the variation of the ambient condition of the sandwich structure to deteriorate the adhesive bonding together the surface plates and the honeycomb core and to reduce the bonding strength of the adhesive and, consequently, the surface plates separates from the honeycomb core.
The water collected in the honeycomb core increases the weight of the sandwich structure, changes the position of the center of gravity of the sandwich structure adversely affecting the dynamic characteristic of the sandwich structure. In some cases, the sandwich structure is heated at a temperature not lower than 100xc2x0 C. when repairing the damaged sandwich structure. If the honeycomb core contains water, the water evaporates to increase the pressure in the sandwich structure when the sandwich structure is heated and, in the worst case, the sandwich structure will burst.
If it is known that the honeycomb core contains water, small holes are formed in the surface plate and the water is drained through the small holes or the sandwich structure is unfastened and the water is removed by suction, which, however, needs time and costs.
When fabricating a sandwich structure having portions of different thicknesses, a honeycomb core must be machined, which requires troublesome work.
When repairing a damaged portion of a sandwich structure by laminating prepreg sheets, heat and pressure must be applied to the laminated prepreg sheets to set the prepreg sheets, which needs special devices. When setting prepreg sheets by applying heat and pressure thereto, pressure and temperature must be strictly controlled, which requires very troublesome work. After completing the repair of the damaged part of the sandwich structure by setting the prepreg sheets, the adhesive bonding the laminated prepreg sheets covering the damaged part to the mother structure of the repaired part must be inspected. Thus, the repair of the damaged sandwich structure needs special devices and much time and work.
Accordingly, it is an object of the present invention to provide a sandwich structure capable of inhibiting the penetration of water therein and having a core that can be easily processed and to provide a method of repairing a sandwich structure by simple work.
According to a first aspect of the present invention, a sandwich structure includes a core having foam plastic core members formed of a closed-cell foam plastic material, and surface plates formed of a fiber-reinforced composite material containing hydrophobic inorganic fibers as reinforcing fibers and covering the opposite surfaces of the core, wherein the foam plastic core members are bonded together with adhesive films that do not foam, and the surface plates are bonded to the core with adhesive films not containing any carrier or a component resin of the fiber-reinforced composite material forming the surface plates.
Since the sandwich structure is provided with the foam plastic core members of a closed-cell foam plastic material, water is unable to collect in the sandwich structure. Since the foam plastic core members are bonded together with an adhesive film that does not foam and the composite surface plates are bonded to the core with an adhesive film not having any carrier or a component resin of the fiber-reinforced composite material forming the composite surface plates, any passages through which water penetrates and diffuses into the sandwich structure are not formed at all. Consequently, the sandwich structure is free from troubles due to the penetration of water into the sandwich structure.
According to a second aspect of the present invention, a sandwich structure includes a core having foam plastic core members of a closed-cell foam plastic material and honeycomb core members, and surface plates formed of a fiber-reinforced composite material containing hydrophobic inorganic fibers as reinforcing fibers and covering the opposite surfaces of the core, wherein a peripheral part of the core is formed of the foam plastic core members arranged so as to surround the honeycomb core members, the foam plastic core members are bonded together with adhesive films that do not foam, and the surface plates are bonded to the core with adhesive films not containing any carrier or a component resin of the fiber-reinforced composite material forming the surface plates.
Since the peripheral part of the sandwich structure through which water is most liable to leak into the sandwich structure is formed of the foam plastic core members, the foam plastic core members are bonded together with an adhesive film that does not foam, and the surface plates are bonded to the core with adhesive films not containing any carrier or a component resin of the fiber-reinforced composite material forming the surface plates, the sandwich structure is free from troubles due to the penetration of water into the sandwich structure. Since the core has the honeycomb core members having strength and rigidity which are generally higher than those of the foam plastic core members, the sandwich structure has a high strength and a high rigidity.
According to a third aspect of the present invention, a sandwich structure includes a core having foam plastic core members formed of a closed-cell foam plastic material and honeycomb core members, and surface plates formed of a fiber-reinforced composite material containing hydrophobic inorganic fibers as reinforcing fibers and covering the opposite surfaces of the core, wherein the core is formed by sandwiching the honeycomb core members between the foam plastic core members and bonding the foam plastic core members to the honeycomb core members with adhesive films not containing any carrier.
Since the core is formed by sandwiching the honeycomb cores between the foam plastic core members and bonding the foam plastic core members to the honeycomb core members with an adhesive film not containing any carrier, water is unable to penetrate into the honeycomb core members and the sandwich structure even if the surface plate is fissured or broken. Since the core has the honeycomb core members having strength and rigidity which are generally higher than those of the foam plastic core members, the sandwich structure has a high strength and a high rigidity. Since the foam plastic core members capable of readily absorbing shocks are contiguous with the surface plates, the sandwich structure is resistant to damaging actions.
According to a fourth aspect of the present invention, a sandwich structure includes a core having foam plastic core members formed of a closed-cell foam plastic material and honeycomb core members, and surface plates formed of a fiber-reinforced composite material containing hydrophobic inorganic fibers as reinforcing fibers and covering the opposite surfaces of the core, wherein portions of the core corresponding to portions of the sandwich structure where thickness varies sharply are formed of the foam plastic core members, portions of the core corresponding to portions of the sandwich structure where thickness varies gradually are formed of the honeycomb core members, and the composite surface plates are bonded to the core with adhesive films not containing any carrier or a component resin of the fiber-reinforced composite material forming the surface plates.
Since the portions of the core corresponding to portions of the sandwich structure where thickness varies sharply are formed of the foam plastic core members capable of being easily processed and of being smoothly processed according to the variation of the thickness, the core can be easily processed and the composite surface plates can be easily bonded to the core.
According to a fifth aspect of the present invention, a sandwich structure repairing method of repairing a sandwich structure including a core having foam plastic core members formed of a closed-cell foam plastic material, and outer and inner surface plates of a fiber-reinforced composite material and bonded to the opposite surfaces of the core with an adhesive film not containing any carrier or a component resin of the composite surface plates includes the step of attaching a support plate to the inner surface of a damaged portion of the outer surface plate with rivets.
Since the support plate is attached to the inner surface of the outer surface plate, the repaired portion of the outer composite surface plate has a smooth surface. Since the support plate is attached to the outer composite surface plate by riveting work using rivets for repair instead of bonding work using an adhesive that requires time and labor, time and labor necessary for repairing the sandwich structure can be greatly reduced.
According to a sixth aspect of the present invention, a sandwich structure repairing method of repairing a sandwich structure comprising a core having foam plastic core members formed of a closed-cell foam plastic material, and an outer surface plate and an inner surface plate formed of a fiber-reinforced plastic material and bonded to the opposite surfaces of the core with adhesive films not containing any carrier or a component resin of the fiber-reinforced composite material forming the surface plates, said sandwich structure repairing method comprising the steps of: putting an outer support plate to an inner surface of a damaged portion of the outer surface plate; and fastening the outer support plate to the outer surface plate with rivets.
When an operator is able to perform repairing work on the side of the inner surface of the sandwich structure and the damage in the outer surface plate is slight, the outer support plate can be attached to the inner surface of the outer surface plate from the inner side of the sandwich structure with the rivets, and the cavity can be filled up with the filling material, and the inner support plate can be easily attached to the inner surface plate.
Preferably, a portion of the inner surface plate and a portion of the core corresponding to the damaged portion of the outer surface plate are removed; the outer support plate is fastened to the inner surface of the damaged portion of the outer surface plate with rivets; a cavity formed by removing the portion of the core is filled up with a filling material; and an inner support plate of a size greater than that of an opening in the inner surface plate formed by removing the portion of the inner surface plate is fastened to the inner surface plate with rivets so as to cover the opening in the inner surface plate.
When an operator is able to perform repairing work on the side of the inner surface of the sandwich structure, the portion of the outer surface plate including the damaged portion is removed to form the opening, the outer support plate is attached to the inner surface of the outer surface plate, the space formed by removing the portion of the core is filled up with the filling material, and the cover plate of the size equal to that of the opening in the outer surface plate is fitted in the opening in the outer surface plate and the cover plate is fastened to the support plate with rivets. Thus, the outer surface of the outer surface plate can be finished in a smooth surface. Since the method employs riveting work using rivets, time and labor for the repair work can be reduced.
Preferably, an opening is formed in the damaged portion of the outer surface plate; a portion of the core and a portion of the inner surface plate corresponding to the opening and a region around an edge of the opening are removed; the outer support plate is fastened to the inner surface of a portion of the outer surface plate around an edge of the opening in the outer surface plate with rivets; a cover plate formed of the same material as that forming the outer surface plate and having a thickness equal to that of the outer surface plate is fitted in the opening and is fastened to the outer support plate with rivets; a cavity formed in the core by removing the portion of the core is filled up with a filling material; and an inner support plate of a size greater than that of an opening in the inner surface plate formed by removing the portion of the inner surface plate is fastened to the inner surface plate with rivets so as to cover the opening in the inner surface plate.
Preferably, a portion of the outer surface plate including the damaged portion is removed to form an opening in the outer surface plate; a portion of the core corresponding to the opening in the outer surface plate and a portion of the core around an edge of the opening in the outer surface plate are removed; the outer support plate is expandable over a region greater than the opening in the outer surface plate; the outer support plate is inserted through the opening in the outer surface plate; the outer support plate is fastened to the inner surface of the outer surface plate with rivets; a cover plate of a size equal to that of the opening formed in the outer surface plate and of a thickness equal to that of the outer surface plate is fitted in the opening in the outer surface plate; the outer support plate is fastened to the support plate with rivets; and a cavity formed by removing the portions of the core is filled up with a filling material.
Since the expandable outer support plate is used, the outer surface of the outer surface plate can be finished in a smooth surface even if repairing work can be carried out only on the side of the outer surface of the sandwich structure.
Preferably, a plurality of the outer support plates ar used; a portion of the outer surface plate including the damaged portion is removed to form an opening in the outer surface plate; a portion of the core corresponding to the opening in the outer surface plate and a portion of the core around an edge of the opening in the outer surface plate are removed; the plurality of the outer support plates are inserted through the opening in the outer surface plate so as to be arranged on the inner surface of a portion of the outer surface plate around an edge of the opening in the outer surface plate; the plurality of the outer support plates are fastened to the inner surface of the outer surface plate with rivets; a cover plate of a size equal to that of the opening formed in the outer surface plate and of a thickness equal to that of the outer surface plate is fitted in the opening in the outer surface plate; the cover plate is fastened to the plurality of the outer support plates with rivets; and a cavity formed by removing the portions of the core is filled up with a filling material.
Since the plurality of the outer upport plates are used instead of the expandable outer support plate, complicated processing work for forming the expandable outer support plate is unnecessary and hence processing work can be reduced.
Preferably, the portions of the core of a foam plastic material corresponding to the opening in the outer surface plate and a region around the edge of the opening in the outer surface plate are removed by shot blasting work.
Since the sandwich structure has the core of a foam plastic material, the portions of the core can be easily removed by shot blasting work, which reduces time and labor necessary for repairing